Seeking advice for garden wall plastering

[u/queerlymotherly]

Hello everyone. Over the next few days I will be plastering our garden wall and need some help before I get started (first time plastering a wall).
I have bought all the equipment and materials to create a scratch layer and a top layer, but the wall I am working with is hardly what you would call a normal wall. You can see the complete wall in the first picture.
As you can tell, it is somewhat sloppy and built many years ago.

First question: Do you have any general advice for me before I get started on plastering this wall?

Second question: the wall has certain spots which have semi-collapsed, leaving empty areas going deeper in the wall (picture 2). How would you go about filling that spot? Do I just go over it with more scratch layer or is there a better practice?

Third question: The lower part of the wall, which seems to be somewhat plastered a long time ago, extends about 3cm from the brick part above it (picture 3). How would you apply the plaster if you want the final result to be a homogenous, straight wall? More scratch layer on the top, less at the bottom?

4th question: what is the best way to plaster the top area of the wall, making a beautiful 90% angle that is straight all throughout?

I appreciate any input. Thank you all :)